In the 1920s Otto Warburg first proposed non-oxidative metabolism of glucose as a unique feature of tumors (Warburg, (1930) Ueber den stoffwechsel der tumoren (London: Constable); Warburg, (1956) Science 123, 309-314). This hypothesis has since caused significant interest and although mechanistic links are still, almost 100 years later, under investigation. A high glucose flux of tumor is today exploited clinically, using PET imaging of 18F-2-deoxyglucose uptake as a diagnostic tool for solid tumors.
Lately, energy processing of cancer cells has been given new attention (e.g. Vander Heiden, et al., 2009, Science 324, 1029). The hypoxic microenvironment and consequential lactate accumulation resulting from altered tumor metabolism are reported predictive for both metastatic potential and therapy resistance, and thus survival of cancer patients (Brown, (1999) Cancer Res. 59, 5863-5870; Walenta& Mueller-Klieser, (2004) Semin. Radiat. Oncol. 14, 267-274; Walenta et al., (2004) Curr. Med. Chem. 11, 2195-2204). Targeting of hypoxic and/or acidotic tumor areas has therefore drawn attention as a complement to anti-proliferative treatments (see e.g. Pan &Mak, (2007) Sci. STKE 381, pe14; Bache et al., (2008) Curr. Med. Chem. 15, 322-338 for reviews).
Known inhibitors of glycolysis include among others 2-deoxyglucose and 2-bromo-puruvate targeting hexokinase (Liu et al., (2001) Biochemistry 40, 5542-5547; Liu et al. (2002) Biochem. Pharmacol. 64, 1745-1751; Xu et al., (2005) Cancer Res. 65, 613-621; Ramanathan et al., (2005) Proc. Natl. Acad. Sci. USA 102, 5992-5997). Fructose-2,6-bisphosphate (F-2,6-P2) plays a regulatory role in glucose metabolism by relieving ATP inhibition of phosphofructokinase-1. The levels of F-2,6-P2 are regulated by the bifunctional enzyme family 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFKFB1-4).
Out of these four isozymes, mainly PFKFB3 and PFKFB4 are of particular interest for playing a role in cancer. Anti-sense treatment against PFKFB3 was shown to reduce tumor growth rate in vivo (Chesney et al., (1999) Proc. Natl. Acad. Sci. USA 96, 3047-3052). Similarly, a decreased anchorage independent growth was shown for siRNA treated fibroblasts (Telang et al., (2006) Oncogene 25, 7225-7234). A link between inflammation and enhanced glycolysis and a possible potential for PFKFB3 inhibitors to act as a anti-inflammatory agents was indicated by a report that the IL-6-STAT3 pathway may enhance glycolysis through the induction of PFKFB3 (Ando et al. J Nippon Med Sch (2010), 77, (2), 97-105). This possibility was further supported by a recent study using a small molecule; 3-(3-pyridinyl)-1-(4-pyridinyl)-2-propen-1-one (3PO), previously shown to reduce F-2,6-P2 synthesis, glucose uptake and proliferation in transformed cells (see below). Telang et al. demonstrated that 3PO attenuates the activation of T cells in vitro and suppresses T cell dependent immunity in vivo, indicating that small molecule inhibitors of PFKFB3 may prove effective as T cell immunosuppressive agents (Telang et al., (2012) Journal of Translational Medicine 2012, 10:95). Moreover, hypoxia is a prominent feature in rheumatoid arthritis (RA) synovium and induces significant changes in the expression of PFKFB3 and PFKFB4 (Del Rey et al., (2010) Arthritis & Rheumatism 62, 3584-3594).
The PFKFB4 protein was reported to be strongly responsive to hypoxia (Minchenko et al., (2004) FEBS Lett. 576, 14-20); Minchenko et al., (2005), Biochemie 87, 1005-1010; Bobarykina et al., (2006), Acta Biochemica Polonica 3, 789-799). US2010/0267815 A1). Minchenko et al. demonstrated an increased expression of PFKFB4 mRNA in malignant breast and colon cancers, as compared to corresponding non-malignant tissue counterparts. Recently, Telang et al. showed decreased levels of F-2,6-P2 and lactate as well as decreased tumor growth following siRNA silencing of PFKFB4 (Telang, S. et al, (2010). Further support for PFKFB4 as a potential target for the development of antineoplastic agents came from a functional metabolic screen that identified PFKFB4 as an important regulator in prostate cancer (Ros et al. (2012) Cancer Discov. 2(4):328-43).
Only a small number of specific inhibitors of the kinase activities of PFKFB3 and PFKFB4 have been identified. In one study, an alkylating inhibitor, N-bromoacetylethanolamine phosphate, was used as a tool to investigate the binding sites of the kinase and phosphatase domains of PFKFB3 and demonstrated to irreversibly inactivate PFK-2 (Sakakibara et al. (1984), J. Bio Chem 259, 14023-14028). The compound is a competitive inhibitor of PFK-2 with respect to F6P but a non-competitive inhibitor with respect to ATP. Analogues of this compound, N-(2-methoxyethyl)-bromoacetamide, N-(2-ethoxyethyl)-bromoacetamide and N-(3-methoxypropyl)-bromoacetamide, have demonstrated in vivo activity with increased survival rate of P388 transplant BDF1 mice (Hirata et al. (2000) Biosci. Biotechnol. Biochem. 64, 2047-2052).
A crystal structure of the PFKFB3*ADP*phosphoenolpyruvate complex was described by Kim et al. (Kim et al. (2007), J. Mol. Biol. 370, 14-26). This paper also described the crystal structures of PFKFB3*AMPPCP*fructose-6 phosphate complex in which β,γ-methylene-adenosine 5′-triphosphate (AMPPCP) constituted a non-hydrolysable ATP-analogue. Recently, small molecule PFKFB3 inhibitors identified by virtual screening were described (Chrochet et al. (2011), Anal. Biochem. 418, 143-148; Seo et al., (2011), Plosone, 9, e24179 & Lee et al. (2012) US 2012/0302631). The identified PFKFB3 inhibitors were shown to reduce the levels of F-2,6-P2, resulting in decreased tumor growth and increased cell death.
A drug-like compound was described (Clem et al. (2008) Mol. Cancer Ther. 7, 110-120; Chesney et al. (2008) WO 2008/156783) where 3-(3-pyridinyl)-1-(4-pyridinyl)-2-propen-1-one (3PO), by computational methods, was identified as a PFKFB3 inhibitor. Administration of 3PO reduced the intracellular concentration of F-2,6-P2, glucose uptake, and growth of established tumors in vivo. Recently, substituted benzindoles were described as inhibitors of PFKFB3. The benzindoles were shown to inhibit proliferation in several cancer cell lines, inhibit glucose uptake as well as to reduce tumor growth in vivo in tumor models (Chand et al. (2011) WO2011/103557A1).